The long term objectives of this research program are to elucidate the neural circuits formed, and neurotransmitters used, by retinal and non- retinal afferents, interneurons, and geniculo-cortical relay cells in the thalamic visual relay, the dorsal lateral geniculate nucleus (LGN). These circuits constitute the morphological and neurochemical substrate for the transfer functions of visual information form the retina to the cortex in the LGN. Continuing with our studies on inhibitory circuits in LGN, the proposed projects will analyze the synaptic relations of two different types of GABAergic interneurons of cat LGN: The type 3 GABA-immunoreactive interneurons in the laminar regions of the geniculate and the GABA- immunoreactive interneurons in the laminar regions of the geniculate and the GABA-immunoreactive interneurons resident in the interlaminar zones of the geniculate. The different location and morphology of these cells suggest that they have different functions in the inhibitory interactions in LGN, the understanding of which requires knowledge of their synaptic relations with other neural elements in this nucleus. Synaptic inputs into these interneurons form retinal and non-retinal afferents, and synaptic outputs of these cells on relay cells or other interneurons, will be studied at electron microscopic level in material double-labeled with the Golgi gold-toning method and with BABA immunocytochemistry using peroxidase-antiperoxidase or immunogold methods. A second main goal of the proposal is to determine the neural circuits established by Y-retinogeniculate axons with relay cells and interneurons and cat LGN. These data will be useful to understand the transfer functions and inhibitory interactions along Y streams in LGN. Y- retinogeniculate axons will be physiologically classified and intracellularly injected with HRP. The synaptic relations of HRP-labeled Y-retinal terminals with GABA+ interneurons and with BABA (-) relay cells will be examined at EM level in serial ultrathin sections, some of which will be treated with a post-embedding GABA immunogold method. The postsynaptic neurons will be partially reconstructed to visualize their morphology, and will be analyzed with respect to innervation divergency by the labeled Y-axon, innervation convergency by labeled and non-labeled retinal terminals, and with respect to synaptic inputs from non-retinal terminals.